IntroductionI decided to make another attempt at building
a tapped-horn, using some of the knowledge I gained from my "Proof of
Concept #2" build. I opted to use the same driver, the
Parts
Express PA310-8 ,
but this time I opted to use a different folding technique, one that was
used for another popular DIY tapped-horn build, the SS15, the main reason
for this being that I needed to come up with a design that was a bit more
transportable than POC#2, which threatened to break the back glass in my SUV
every time I went over a bump in the road. I also decided to build it partly
out of 12mm ply to reduce the weight of the box.

To assist with coming up with the
best dimensions for the new tapped-horn, I put together a spreadsheet that
would allow me to adjust the various dimensions until I came up with a
smooth expansion curve. The spreadsheet then gave me the appropriate
parameters to insert into HornResp to simulate what the results would be
like. By varying the dimensions in the spreadsheet and updating the
simulation accordingly, I was able to come with a decent-looking alignment
in a reasonably-sized box.

The HornResp parameters are as
follows:

The predicted performance is as
follows:

The required panel
dimensions are as follows:

Not included in the above list are
the panels required to brace the enclosure properly.

The required layout is as follows:

I then proceeded to build the box
(see images below). Ingredients: one sheet of 4 x 8 18mm ply, one
sheet of 4 x 8 12mm ply, a box of
drywall screws, wood-glue, a few bricks (to act a temporary table), some
mounting screws for the driver and of course the driver. This time around I
left no removable panels, as it's possible to mount the driver in the box
without having to remove any panels. This build was a bit more difficult
than my previous POC build due to the number of panels involved, the
bracing, and the different thicknesses of ply used.

The image below shows the box
partway though the build. You can see where I've inserted bracing in
almost all sections of the horn. Not visible in the image is an
additional bracing panel mounted right behind the panel that's facing the
driver.

The image below shows what I did for the
baffle, where I used two pieces of 12mm ply bonded together, and cut so that
the driver is countersunk into the hole. Doing it this way ensures
that the driver is properly positioned over the cutout, as any minor leaks
in this area will result in a considerable loss in low-frequency output.

The image below shows what the built box looks
like in my car. It's wider than my previous tapped-horn, but it is not
as high not as deep. I can actually transport it now without having to
put down the rear seats.

Results:
The tapped-horn seems to be performing as expected, with that "effortless
sound" that characterized my previous proof-of-concept builds. Peak output is
quite decent considering the capabilities of the driver that I'm using in it
but, as per previous tapped-horn build, a steep low-pass filter needs to be
used to address the out of band noise. Based on my listening
impressions, the results to me appear to be equivalent, if not superior to,
what was achieved with my previous POC build, and this is before I've done
any fine-tuning, like perhaps adding some stuffing to see if the peak around
160 Hz could be reduced.

The impedance curve of the built
system (see below) suggests that further bracing may not be required, as the
three impedance peaks are well-formed (as compared to my previous build,
where the third peak was almost non-existent until I properly braced the
box).

The image below shows the measured
frequency response (in red) , compared to the HornResp predictions (in
brown). Also shown is the measured response with 48dB/oct filters applied at
30 Hz and 120 Hz (blue). The measured results suggest that Fb is between
38~39 Hz, an almost perfect match for the simulation. I suspect that it's
slightly lower because the driver's structure is blocking the mouth a bit.

The images below shows one
my "Blastorama" speakers on top of the box, which should give a good
indication of its size. The second picture was taken after the subwoofer was
painted flat black and the mouth was reinforced with a few metal brackets.

DSP

The response of a tapped-horn can
be cleaned up significantly with the use of DSP, and I strongly suggest that
if you plan on building one, you consider including a DSP solution in the
signal path to get the most out of it.

For example, here's the raw
response of the POC#3: :

...here's the response with DSP
used to flatten the response between 40 Hz to 200 Hz:

...here's the final response with
DSP used to apply a 48dB/oct filter at the low end (to control
excursion below the resonance frequency of the tapped-horn) and at around
100 Hz (to blend in with the main drivers):

.The combination of the EQ and
X-OVER results in a much smoother response and easier blending with the main
speakers. The DSP requirements to accomplish this could be met by using
something like the Behringer iNuke 3000DSP amplifier to power the
tapped-horn.